FAST discovers lightest binary neutron star system

China's Five-hundred-meter Aperture Spherical radio Telescope (FAST), the world's most sensitive single-dish radio telescope, has discovered a binary neutron star system with an extremely short orbital period and the lowest total mass ever recorded, providing a new laboratory for studying matter under extreme conditions, testing theories of gravity and exploring the origins of heavy elements in the universe.

The system, which contains a pulsar named PSR J1856−0039, completes one orbit every 2.36 hours — the second-shortest orbital period ever recorded for a binary neutron star system — and has a combined mass of 2.488 times that of the sun. It is the lightest binary neutron star system confirmed to date, according to findings published Tuesday in the journal Physical Review Letters, which selected the study as an editors' highlight.

Neutron stars are the extremely dense remains of massive stars that have exploded as supernovae. Although they are only about 20 kilometers across, they can have more mass than the sun and rotate rapidly, with pulsars emitting regular radio signals that can serve as highly precise cosmic clocks. When two neutron stars orbit each other, they form an exceptionally rare type of system. Only about 30 such systems have been firmly identified so far, making them valuable natural laboratories for studying extreme physics and gravity.

Binary neutron star systems are also important for understanding how some of the universe's heaviest elements are formed. When two neutron stars eventually collide, they release gravitational waves — ripples in space and time — and can produce heavy elements such as gold and platinum through a process in which atomic nuclei rapidly capture neutrons. Systems with particularly short orbital periods are especially valuable because their close orbits produce stronger effects predicted by Einstein's theory of general relativity and cause the stars to move toward an eventual merger more quickly.

To search for such systems, FAST uses an L-band receiver that can observe 19 areas of the sky simultaneously, allowing it to detect extremely faint radio pulses from distant objects. Using a snapshot survey mode developed by the research team, scientists efficiently scanned the Milky Way's stellar disk — the broad, densely populated region of the galaxy where most pulsars are found.

The survey has discovered nearly 900 new pulsars, with PSR J1856−0039 standing out for its scientific value.

"In this system, the visible pulsar weighs about 1.30 solar masses, while its companion is about 1.19 solar masses — making both among the lightest neutron stars ever detected and close to the theoretical minimum," said Han Jinlin, a professor at the National Astronomical Observatories of the Chinese Academy of Sciences who led the study. Han said the system could provide important constraints on the poorly understood physics of supernova explosions.

The system's extremely tight orbit also provides an opportunity to test Einstein's theory of general relativity under intense gravitational conditions. FAST observations have detected several effects predicted by the theory, including the gradual rotation of the system's elliptical orbit, a shift of light toward longer, redder wavelengths as it escapes the strong gravitational field, and a slight slowing of the passage of time in the stronger gravitational field.

The two neutron stars are also losing energy by emitting gravitational waves, causing their orbit to gradually shrink. The measured rate of this orbital decay closely matches Einstein's prediction. Researchers estimate that the two stars will merge in about 82 million years, most likely forming a heavier neutron star rather than collapsing directly into a black hole. The finding could help scientists better understand the interior of neutron stars and the origins of heavy elements in the universe.

The system's combination of a small tilt in its orbit and an ultrashort orbital period also creates a rare opportunity to detect frame dragging, an effect in which a rapidly spinning object slightly drags the surrounding space and time along with its rotation.

"In simple terms, a spinning neutron star stirs the spacetime around it. Among known binary neutron star systems, only one or two such systems are promising for such a measurement," Han said.

He added that long-term, high-precision monitoring with FAST could determine how the pulsar's mass is distributed as it spins, providing clues about its internal structure and helping scientists better understand how gravity behaves under extreme conditions.